365 Days of Climate Awareness 352 – AR6, Vol. 3, Chap. 8: Urban Systems and Other Settlements

Cities have typically been the result of growing wealth in a society. Now they represent opportunities to decarbonize at scale, and just as importantly, ahead of anticipated rapid 21^st century growth in developing countries, where with current methods, per-capita greenhouse gas (GHG) emissions are likely to increase. It is estimated that in 2015, cities around the world were responsible for 24.5 GtCO₂-eq (gigatons of CO₂ equivalent), or 62% of the world total. The 100 largest urban areas on the planet account for about 18% of all GHG emissions.

All illustrations from the IPCC 6th Assessment Report, Vol. 3, Chap. 8. Urbanization versus Gross National Incomes (GNI). 

2018 world population: rural versus urban.

The urban share of GHG emissions increased from 56% to 62% between 2000 and 2015, though this increase was unevenly distributed around the world, occurring more in developing regions where cities have been rapidly growing.  The same was observed in per-capita urban emissions, where the largest increase occurred in developing countries as they built out their infrastructure. Both trends are expected to continue, and represent one of our largest opportunities for mitigation.

2015 urban per capita GHG emissions. 

2050 and 2100 forecasts for urban emissions. [SSP1: Sustainability (Taking the Green Road); SSP2: Middle of the Road; SSP3: Regional Rivalry (A Rocky Road); SSP4: Inequality (A Road divided); SSP5: Fossil-fueled Development (Taking the Highway)] 

Many cities are built in vulnerable locations, at low elevation, along coasts and rivers. Furthermore, most developing nations are themselves particularly vulnerable to global warming, being in or near the tropics. For these reasons mitigation and adaptation strategies must be combined, achieving not only low-carbon energy sources and efficient uses, but also employing flood protection and more ecologically-minded freshwater delivery systems.

Mitigation potentials to 2100. 

Decarbonizing cities will require large-scale transformations of existing systems, in three main efforts:

1.   Reducing energy consumption in all sectors;
2.   Electrifying as much as possible and switching to zero-carbon energy sources;
3.   Enhancing carbon uptake (i.e. planting trees).

Cities cannot effectively decarbonize by focusing only on emissions within their city limits. Taking supply chains and out-of-city residents into account, efficiency and no-carbon energy sourcing must reach beyond the city’s limits to be effective.

Emissions mitigation scenarios. [SSP1: Sustainability (Taking the Green Road); SSP2: Middle of the Road; SSP3: Regional Rivalry (A Rocky Road); SSP4: Inequality (A Road divided); SSP5: Fossil-fueled Development (Taking the Highway)] 

It will be easier to build low- and no-carbon systems in still-developing cities than to convert existing metropolitan areas, though the problem will be funding in those typically poorer (still developing) nations and the political and social will to dramatically change development plans. Restructuring existing cities, even where money to invest exists, will require huge political and social efforts. But the potential in creating more ecologically-minded green and blue spaces within cities, and replacing existing infrastructure with more efficient and lower-carbon sources, could reduce GHG emissions by 23-26% by 2050.

Tomorrow: buildings.

Be brave, be steadfast, and be well.

IPCC 6th Assessment Report, Vol. 3, Chap. 8